In the AMLCD and OLED display field, thin film transistors (TFTs) based on poly-crystalline silicon are preferred because of their ability to transport electrons more effectively. Poly-crystalline based silicon transistors (p-Si) are characterized as having a higher mobility than those based on amorphous-silicon based transistors (a-Si). This allows the manufacture of smaller and faster transistors, which in turn allows the production of brighter and faster displays.
One problem with p-Si based transistors is that their manufacture requires higher process temperatures than those employed in the manufacture of a-Si transistors. These temperatures range from 450 to 600° C. compared to the 350° C. peak temperatures employed in the manufacture of a-Si transistors. At these temperatures, most AMLCD glass substrates undergo a process known as compaction. Compaction, also referred to thermal compaction, is an irreversible dimensional change (shrinkage or expansion) in the glass substrate due to changes in the glass' fictive temperature. This hypothetical temperature is defined as that temperature at which the glass structure is in thermal equilibrium. As such, the fictive temperature of a glass is a measure of the glass' viscoelastic response to its previous thermal history. Since the manufacture of these transistors require the sequential alignment of 4-6 layers to micron tolerances through photolithography processes, this compaction behavior is problematic. Compaction is dependent upon both the inherent viscous nature of a particular glass composition (as indicated by its strain point) and the thermal history of the glass sheet as determined by the manufacturing process. Higher temperature processing (such as required by low temperature p-Si TFTs) may require the addition of an annealing step to the glass substrate to ensure the glass has sufficient thermal stability, i.e. undergoes minimal compaction.
There are two approaches to correct, or minimize, the compaction behavior in glass. The first is to thermally pretreat the glass to create a fictive temperature similar to the one the glass will experience during the p-Si TFT manufacture. However there are several difficulties with this approach. First, the multiple heating steps employed during the p-Si TFT manufacture create slightly different fictive temperatures in the glass that cannot be fully compensated for by this pretreatment. Second, the thermal stability of the glass becomes closely linked to the details of the p-Si TFT manufacture, which could mean different pretreatments for different customers. Finally, pretreatment adds to processing costs and complexity.
Another approach is to slow the kinetics of the compaction response. This can be accomplished by raising the viscosity of the glass. Thus if the strain point of the glass is much greater than the process temperatures to be encountered (>˜200-300° C.), compaction is minimal. The problem with this approach, however, is how to make such high strain point glass substrates cost effectively. For example, the fusion process, which is highly valued in these applications for its ability to make very smooth surfaces, requires a glass that is very stable with respect to devitrification. This requirement precludes the manufacture of “fragile” type glasses on a fusion draw. Fragile glasses are glasses having a steep viscosity curve, for example, very high strain point (for minimal compaction) and low melting temperature (for easy melting). See for reference, C. A. Angell, “Spectroscopy Simulation and Scattering, and the Medium Range Order Problem in Glass”, J. Non-Cryst. Solids, 73 (1985) 1-17. These types of glasses tend to be more prone to devitrification (formation of a crystalline phase in the glass, and, as a result, tend to be less compatible with the forming requirements imposed by the fusion process.
What is needed in the art are low compaction p-Si glass substrates. It would be desirable to manufacture a lower compaction (high strain point) glass without having to significantly raise the thermal capability of drawing process (e.g., the fusion drawing). To address this need, described herein are laminated glass articles composed of a skin and a core, wherein the skin is composed of a low compaction and high strain point glass.